Making a Coherer. How to Adjust the Instruments.
MANY experimenters desire some device by which wireless signals may be
made clearly audible without the necessity of holding a telephone
receiver to the ear. It is not possible for the amateur to make such an
arrangement which will receive signals over long distances. It is,
however, possible to arrange a "coherer" so that signals coming from a
nearby station may be indicated by the ringing of a bell.
[Illustration: FIG. 162.—The Coherer Details.]
Such a "coherer" outfit will receive messages coming within a radius of
one mile and may also be used for demonstrating the principles of
wireless telegraphy when both the receiving and transmitting station are
located in the same room or building.
In order to secure these results it will be necessary to substitute a
device called a "coherer" for the detector in the receiving apparatus.
A coherer consists essentially of a few metal filings between two metal
plugs. The filings are made part of a circuit which includes a relay and
a battery. Under ordinary conditions the filings lie loosely together
and their resistance to an electric current is so great that they will
not allow enough energy to flow to operate the relay. If, however, the
two plugs are connected to the aerial and ground so that the currents of
the incoming wireless signals also pass through the filings, they will
suddenly "cohere" or stick together and permit sufficient current to
pass to operate the relay.
[Illustration: FIG. 163.—The Complete Coherer.]
A "tapper," or as it is also sometimes called a "decoherer," is also
arranged in the circuit so as to automatically shake the filings up and
restore them to their loose state as soon as each signal is received.
The decoherer consists simply of an ordinary bell mounted so that the
hammer will strike the coherer and jar it.
The first thing required in order to make a coherer is a pair of double
binding posts. These should be mounted on a wooden base six inches long
and four inches wide as shown in Figure 163.
A piece of glass tubing about one and one-half inches long and
one-eighth of an inch in diameter inside together with some brass rod
which will just slip into the tube tightly will be required.
Cut two pieces of the brass rod one and three-quarters inches long and
face off the ends perfectly square and smooth.
Slip these rods through the upper holes in the binding posts and into
the glass tube as shown in Figure 163. Be sure that everything lines up
perfectly and then remove one of the plugs.
Before putting the plug back in place put some nickel and silver filings
in the tube so that when the rods are pushed almost together, with only
a distance of three-sixty-fourths to one-sixteenth of an inch between,
the filings will about half fill the space.
[Illustration: FIG. 164.—Pony Type Relay.]
The filings are a mixture of silver and nickel and must be very
carefully prepared. The mixture should be nine-tenths nickel and
one-tenth silver. The filings may be secured by holding a five cent
piece over a clean sheet of paper and filing it with a coarse file. The
silver filings are made in the same way by using a ten cent piece or a
quarter in place of the nickel.
Do not use the fine dust and powder which is formed, but only the coarse
filings.
The success of the coherer will depend upon its adjustment and it will
probably be necessary for the beginner to experiment considerably to
find out just the right amount of filings to place in the tube and how
far apart the brass plugs should be placed.
The decoherer is made by removing the gong from an old electric bell and
mounting the bell on the base in the position shown in Figure 163. The
hammer should be bent so that it will tap the coherer lightly when a
battery is connected to the bell binding posts.
The only thing required to complete the apparatus is a relay.
A relay of the "Pony" type such as that shown in Figure 164 will serve
for ordinary work. It should be wound to at least 150 ohms, for to a
certain extent, the higher the resistance, the more sensitive it will
be.
When it is desired to secure the maximum possible distance with the
coherer a "polarized" relay will be necessary. A polarized relay is much
more sensitive than the ordinary sort. The polarized relay should be
wound to 1000 ohms.
[Illustration: FIG. 165.—Connections for the Receiving Set.]
Figure 165 illustrates how the coherer, decoherer and relay should be
connected. Two wires connected to the coherer posts lead to the magnets
of the relay in series with a single dry cell. The contact posits of the
relay are connected to the decoherer in series with another dry cell.
When wireless signals pass through the relay they will cause the
resistance to drop and allow sufficient current from the dry cell to
pass through the coherer to energize the magnets of the relay and draw
the armature in, thus closing the decoherer circuit. When the armature
of the relay is drawn in towards the magnets the relay contacts are
brought together. This closes the decoherer circuit and starts the
hammer to tapping the coherer, thus restoring the filings to their
former loose state. The decoherer will tap the coherer as long as the
incoming signals continue. By breaking the signals up into dots and
dashes of the telegraph code a message may be easily transmitted and
read from the tapping of the decoherer.
[Illustration: FIG. 166.—Coherer, Decoherer and Relay Connections.]
The best results are secured from a coherer outfit if it is connected to
the aerial and ground by substituting it for the detector in a circuit
employing a double slide tuning coil and a fixed condenser. The two
wires that would ordinarily be connected to the detector terminals are
simply connected to the coherer posts.
When the apparatus is to be used for demonstration or experiment and the
transmitting and receiving stations are both located in the same room or
house it will not be necessary to use an aerial and ground but merely to
attach two copper wires about 18 or 24 inches long to the opposite sides
of the coherer as shown in Figure 163.
In such a case the transmitter should be arranged as in Figure 167.
A spark gap made especially for this purpose is illustrated in Figure
168. Some of the details of the gap are shown in Figure 169.
[Illustration: FIG. 167.—How the Transmitter is Connected.]
The ball may be made in the form of a complete sphere or else consist
simply of half of one as illustrated. It should have a radius of about
five-eighths of an inch and arranged so that it may be mounted upon the
end of a "catch-wire."
[Illustration: FIG. 168.—The Complete Spark Gap.]
The "catch-wires" are mounted in two standards similar in construction
to a large binding post three and one-quarter inches high. The standards
should be mounted by screwing them on top of the secondary terminals of
the spark coil. The spark balls should be adjusted so that the distance
between them is not over one-eighth of an inch.
The coherer apparatus will probably require to be very accurately
adjusted before it will work properly.
Patience and a little experience will soon enable one to overcome the
difficulties and to put the apparatus in sensitive condition without
much trouble.
[Illustration: FIG. 169.—Details of Spark Gap.]
The coherer will be found to be the most sensitive when only a very few
filings are used.
The adjustment desired in the relay is the one in which the armature is
given the smallest possible amount of motion and the spring which pulls
the armature away from the magnets, the least tension.
The best method of finding the most sensitive adjustment for the
apparatus is to place the transmitting and receiving stations only a few
feet apart from each other in the same room. After you learn how to
adjust the coherer and relay properly you will then find that you can
move the transmitter quite a distance away from the coherer and it will
still operate.
Of course a coherer outfit is not very satisfactory and reliable for the
regular reception of wireless signals even over short distances because
of its irregular working. It is, however, very useful for sort of a
calling or signal outfit. It may be connected to the tuning coil by
means of a double-pole, double-throw switch and left in the circuit so
as to indicate by the working of the tapper when any of the other
stations in the near neighborhood are sending. The detector can then be
quickly inserted in the circuit in place of the coherer by throwing the
switch in the opposite direction and the signals read in the telephone
receiver.